1. Field of the Invention
This invention relates to hemming sheet metal and more particularly to a hemming apparatus and method for forming a generally flat hem between two metal panels. Even more particularly, this invention relates to a roller-type hemming apparatus for edge hemming vehicle closure panels, such as hemming door, hood, trunk, and deck lid panels.
2. Description of the Prior Art
In the automotive industry, hemming machines are conventionally used to attach two metal panels together. These metal panels include, for example, the metal panels to form the automotive hood, door panels, quarter panels, and the like.
Hemming machines are disclosed in the patent prior art, including U.S. Pat. No. 5,267,387, issued Dec. 7, 1993 to Sawa; U.S. Pat. No. 6,425,277, issued Jul. 30, 2002 to Wiens; U.S. Pat. No. 6,983,633, issued Jan. 10, 2006 to Holmgren et al.; and U.S. Pat. No. 7,152,447, issued Dec. 26, 2006 to Toeniskoetter; and U.S. Patent Publication No. 2005/0229666, published Oct. 20, 2005 to Toeniskoetter.
A conventional roller hemming apparatus is mounted to a multi-axis controllable robot and may include a hem roller carried by a support. The roller hemming apparatus is adapted for hemming a bent portion of a workpiece, such as a door panel, which is positioned on a hemming die. The workpiece is hemmed by rolling the hem roller, under pressure, along the bent portion. The conventional roller-type hemming apparatus is used for continuous hemming along the contour edge of the workpiece.
In order to achieve good hemming with a conventional hemming apparatus, the robot must move the hem roller along the edge of the workpiece at a constant distance from the hemming die. However, a robot is not comprised of a perfectly rigid body. Therefore, when the hem roller is positioned against the edge of the workpiece, the robot may be deflected by its own resiliency or by a resistive force exerted by the edge of the workpiece itself. This results in a fluctuation of the pressing force applied by the hem roller onto the edge of the workpiece, and therefore imperfect hemming of the workpiece.
Various approaches to compensate for fluctuating pressing force have been proposed. For example, springs have been used. However it has been observed that springs alone have proven to be inadequate.
In addition, traditional hemming rollers having a single roller only enable the user to hem simplistic workpiece such as those having open areas. However single roller hemming apparatuses are not capable of hemming a workpiece with complex configurations.
Additionally, certain workpieces having complex geometry may be difficult to hem. It is costly to reconfigure robot-controlled hemming apparatuses because it is time consuming with respect to labor expenses. In addition, customized tooling required to reconfigure the mounting of the robot is expensive. To overcome this problem, some roller hemming apparatuses provide two rollers on a common spindle. The first roller is used for the actual hemming, while the second roller is used to guide the first roller relative to the workpiece.
Additionally, as work progresses, oftentimes the circumference of the hemming roller may become nicked, thus resulting in an imperfect hemming.